1. Field of the Invention
The present invention relates to a method of controlling the luminance of a plasma display panel that represents gradation using a subfield method or the luminance of a display panel of a digital micromirror device and others, particularly relates to a luminance control method of varying display luminance according to the luminance level of a displayed image.
2. Description of the Prior Art
For an example of a display device using a subfield method, a plasma display panel will be described below.
Referring to the drawings, a conventional type plasma display panel, its driving method and its luminance control method will be described below.
FIG. 1 is a partial sectional view showing the conventional type plasma display panel.
Two front and back substrates 1a and 1b respectively made of glass are provided to the plasma display panel.
A transparent scanning electrode 2 and a sustain electrode 3 are formed on the substrate 1a and a bus electrode 4 is arranged to reduce the resistance values of these electrodes so that it is overlapped with the scanning electrode 2 and the sustain electrode 3.
A first dielectric layer 9 covering the scanning electrode 2 and the sustain electrode 3 is also provided and a protective layer 10 made of magnesium oxide and others for protecting the dielectric layer 9 from discharge is formed.
A data electrode 5 extended perpendicularly to the scanning electrode 2 and the sustain electrode 3 is formed on the substrate 1b. 
A second dielectric layer 11 covering the data electrode 5 is also provided.
A partition 7 extended in the same direction as the data electrode 5 for delimiting a display cell to be a unit of display is formed on the dielectric layer 11.
Further, a phosphor layer 8 that converts ultraviolet rays generated by the discharge of gas to visible light is formed on the side of the partition 7 and on the surface on which no partition 7 is formed of the dielectric layer 11.
Space put between the substrates 1a and 1b and partitioned by the partition 7 is discharged space 6 in which discharged gas made of helium, neon or xenon, or mixed gas of these is filled.
In the plasma display panel configured as described above, surface discharge 100 occurs between the scanning electrode 2 and the sustain electrode 3.
Next, various selective display operations of a display cell will be described.
FIG. 2 is a time chart showing a voltage pulse applied to each electrode in a conventional type driving method.
As shown in FIG. 2, a period A is a priming period for facilitating the occurrence of discharge in a succeeding selective operational period, a period B is the selective operational period for selecting turning on/off display in each display cell, a period C is a sustain period for enabling display discharge in selected all display cells and a period D is a sustain elimination period for halting the display discharge.
In the conventional type driving method, the reference potential of a surface electrode formed by the scanning electrode 2 and the sustain electrode 3 is equivalent to sustain voltage Vs for maintaining discharge in the sustain period C.
Therefore, for the scanning electrode 2 and the sustain electrode 3, higher potential than the sustain voltage Vs is represented as positive and lower potential is represented as negative.
The electric potential of the data electrode 5 is 0 V.
First, in the priming period A, a positive sawtooth priming pulse Pps is applied to the scanning electrode 2 and simultaneously, a negative rectangular priming pulse Ppc is applied to the sustain electrode 3.
The peak value of the priming pulses is set to a value that exceeds discharge starting threshold voltage between the scanning electrode 2 and the sustain electrode 3.
Therefore, the voltage of the sawtooth priming pulse Pps rises by applying the priming pulses Pps and Ppc to each electrode and when voltage between both electrodes exceeds the discharge starting threshold voltage, weak discharge occurs between the scanning electrode 2 and the sustain electrode 3.
As a result, a negative wall charge is generated on the scanning electrode 2 and a positive wall charge is generated on the sustain electrode 3.
A negative sawtooth priming elimination pulse Ppe is applied to the scanning electrode 2 next to the application of the priming pulse Pps.
At this time, the electric potential of the sustain electrode 3 is fixed to the sustain voltage Vs.
The wall charges generated on the scanning electrode 2 and the sustain electrode 3 are erased by the application of the priming elimination pulse Ppe.
The adjustment of the wall charge for satisfactorily executing operation in the next process such as selective operation and sustain discharge is included in the elimination of the wall charges in the priming period A.
Next, in the selective operational period B, after all scanning electrodes 2 are once held base potential Vbw, a negative scanning pulse Pw is sequentially applied to each scanning electrode 2 and a data pulse Pd according to display data is applied to the data electrode 5.
For this while, the sustain electrode 3 is held positive potential Vsw.
The ultimate potential of the scanning pulse Pw and the data pulse Pd is set so that voltage between opposite electrodes formed by the scanning electrode 2 and the data electrode 5 does not exceed the discharge starting threshold voltage when an either pulse is applied and exceeds the discharge starting threshold voltage when both pulses are superposed.
The electric potential Vsw of the sustain electrode 3 in the selective operational period B is set so that it does not exceed the discharge starting threshold voltage between the scanning electrode 2 and the sustain electrode 3 even if the scanning pulse Pw is superposed.
Therefore, in only a display cell to which the data pulse Pd is applied at the same time as the application of the scanning pulse Pw, opposite discharge occurs between the scanning electrode 2 and the data electrode 5.
At this time, as there is potential difference by the scanning pulse Pw and Vsw between the scanning electrode 2 and the sustain electrode 3, discharge is triggered by the opposite discharge and also occurs between the scanning electrode 2 and the sustain electrode 3.
This discharge is writing discharge.
As a result, in a selected display cell 12, a positive wall charge is generated on the scanning electrode 2 and a negative wall charge is generated on the sustain electrode 3.
Afterward, in the sustain period C, the sustain pulse Pps the peak value of which is equivalent to the sustain voltage Vs and each phase of the sustain pulse applied to each scanning electrode and the sustain pulse applied to each sustain electrode of which is inverted is applied to all scanning electrodes 2 and all sustain electrodes 3.
The sustain voltage Vs is set to voltage at which discharge occurs in case wall voltage generated on the surface electrode by the writing discharge in the selective operational period B is superposed on the sustain voltage Vs and at which surface electrode potential does not exceed the discharge starting threshold voltage and no discharge occurs in case such a wall charge is not superposed.
Therefore, in only the display cell 12 in which writing discharge occurs in the selective operational period B and a wall charge is generated, sustain discharge for display occurs.
In the succeeding sustain elimination period D, the voltage of the sustain electrode 3 is fixed to the sustain voltage Vs and a negative sawtooth sustain elimination pulse Pe is applied to the scanning electrode 2.
In this process, the wall charge on the surface electrode is erased and control is returned to an initial state, that is, a state before the priming pulses Pps and Ppc are applied in the priming period A.
The adjustment of the wall charge for satisfactorily executing operation in the next process is also included in the elimination of the wall charge in the sustain elimination period D.
In addition to the method in which the selective operational period and the sustain period are separated in time, a driving method in which these operations are mixed is also adopted, however, from the viewpoint of an individual display cell, the methods are similar in that the selective operational period is provided after the priming and next, the sustain period is provided.
A subfield method is used for the gradation display of the plasma display.
The reason is that in an AC-type plasma display, it is difficult to vary voltage for display luminance and for luminance modulation, the frequency of emission is required to be varied.
According to the subfield method, one image having gradation is analyzed into plural binary display images, they are continuously displayed at high speed and the one image is reproduced as a multi-gradation image by visual integral effect.
FIG. 3 schematically shows a part of a circuit for converting an analog television vide signal to a signal for driving the plasma display panel.
As the plasma display has no gamma characteristic in output, the correction of an output level is first made in a gamma correction circuit.
Next, the luminance level of each color of RGB is converted to a digital signal by an A/D converter.
The conversion is made every eight bits for normal full color display.
Next, the luminance level of each color of RGB is further converted to a subfield selecting signal in a subfield coding circuit.
For example, in case an eight-bit image having 256 gradations is represented in eight subfields, a video signal is digitized to be a binary code acquired by representing image luminance signal data at the ratio of 1:2:4:8:16:32:64:128, and a subfield according to the number of sustain cycles in which luminance according to each gradation is given is allocated.
The number of sustain cycles of each subfield is adjusted so that in a subfield SF1 at the head, display at the least luminance is made, in order, in SF2, display at luminance equivalent to the double in SF1 is made and in SF8, the most luminance is given.
Hereby, a subfield is selected according to the gradation level of each discharge cell and full color display is realized.
In an actual plasma display for full color display, when an animation is displayed, a false contour in an animation proper to the subfield method is caused.
The principle of the occurrence of a false contour in an animation will be described below.
FIG. 4 is a time chart showing a state of the emission of a cell for explaining the principle of the occurrence of the false contour in an animation.
In FIG. 4, the x-axis shows time and the y-axis shows continuous display cells.
For gradations, a case that 64 gradations represented by six bits are represented in six subfields in a weighted binary code will be briefly described below.
A unit of display on a time base is a field and each field is divided into six subfields differently weighted.
In FIG. 4, a case that an image which includes 31 and 32 gradations and the luminance of which smoothly varies move on the screen of the plasma display panel described above is represented.
The subfields shaded in FIG. 4 are emitted subfields and a mark of ▴ shows the center of the gravity of emission which is a mean position of emission in a field.
A number on the side of the center of the gravity of emission denotes a value of the gradation of the subfield.
As shown in FIG. 4, as a field advances from m to m+2 via m+1, an image moves in a direction from n+3 to n via n+2 and n+1.
At this time, in a cell n+1, as a gradation varies from 31 (111110) to 32 (000001) from the field m to the field m+1, the center of the gravity of emission moves from a front half to a rear half in the field (in this case, a subfield in which emission occurs is represented as 1 and a subfield in which no emission occurs is represented as 0).
At this time, in the rear half of the field m and the front half of the field m+1, no emission occurs and a non-display period for one field occurs as encircled by a dotted line.
The non-display period moves with the movement of an image and is recognized as if a black dot (or a black line) moved.
In case a direction in which an image moves is reverse, a full-display period (not illustrated) is recognized as if a white dot (or a white line) moved instead.
As described above, a phenomenon that a contour that does not exist originally in an image appears is called a false contour of an animation.
To prevent the occurrence of such a false contour of an animation, it is effective to avoid the rapid change of the center of the gravity of emission in a field.
For example, a method of increasing the number of subfields so that the number is more than the number of gradation bits and weighting each subfield so that redundancy different from a binary is added is used.
In this case, to realize the display of 256 gradations, the number of subfields exceeding 9 is required.
Such technique is disclosed in Japanese published unexamined patent application No. Hei 10-153982 (which corresponds to U.S. Pat. No. 6,323,880B1 issued on Nov. 27, 2001) for example.
As the luminous efficiency of the plasma display is not high so much, large power is required in case the whole panel is light such as a case of total white display, and a problem of consumed power and a problem of the heat of the panel and a circuit occur.
Therefore, the control of luminance for clear display in which the luminance of full white display is reduced and peak luminance in case the average luminance of the screen is low is enhanced is adopted in the plasma display.
The control of luminance is a method of detecting APL which is an average luminance level of the whole screen and varying the number of sustain discharge cycles of each subfield according to it.
That is, in case APL is low, the number of sustain discharge cycles for one field is increased to realize the display of high luminance and in case APL is high, the number of sustain discharge cycles for one field is reduced to reduce power consumption by emission.
Table 1 shows relation between APL and the number of sustain cycles of each subfield in case 256 gradations are represented by 12 subfields.
In this example, APL consists of 4 steps, the lowest level is APL 0 and a state close to total white is APL 3.
In the state of total white, the number of sustain cycles is 255 even if a luminance level is 255, which is the maximum luminance level. On the other hand, and the number of sustain cycles of a luminance level 255 is 1020 at APL 0 at which peak luminance is realized, sustain pulses of the quadruple number are applied, compared with the number of sustain pulses applied in full white display and peak luminance close to the quadruple of the luminance of full white is realized.
TABLE 1APLSF1SF2SF3SF4SF5SF6SF7SF8SF9SF10SF11SF12Total312481216212632384451255224816243242526476881025101361224364863789611413215376504816324864841041281521762041020
The maximum power as a display is in total white display and in case APL is low, peak luminance can be increased without increasing maximum consumed power.
There are various methods of detecting APL, however, in the case of the plasma display, luminance data is based upon a digital signal and APL can be easily detected by simple digital signal processing.
The number of sustain cycles of each subfield corresponding to each APL can be easily set using a look-up table (LUT).
In the meantime, relation between the luminance level of each pixel and coding for selecting a subfield is uniquely defined independent of APL and is set using LUT and others.
A luminance control method of controlling the number of sustain cycles based upon such information corresponding to an average luminance level of an image, reducing maximum power consumption and increasing peak luminance is called a power saving method and a peak luminance increasing method (PLE) and is also disclosed in Japanese published unexamined patent application No. 2000-322025 for example.
In this specification, the method is called PLE.
However, the luminance control method of the conventional type display panel has the following problems.
The luminance of a plasma display panel has been enhanced year by year and a representable minimum luminance level has been also enhanced according to it.
In such a case, when luminance is controlled using PLE, when luminance is controlled using PLE, a minimum luminance level in case APL is low has a large value.
FIG. 5 shows relation between each luminance level at APL 0 and at APL 3 and actual luminance.
FIG. 5 shows a state of a relatively low luminance level.
As clear from FIG. 5, a step of the variation of luminance at APL 0 is quadruple, compared with that at APL 3.
For example, if a minimum luminance level representable as a plasma display panel, that is, a luminance level in case the number of sustain cycles is one is 1 cd/m2, a minimum luminance level at APL 0, that is, a step of the variation of luminance is 4 cd/m2.
An image of APL 0 is often a relatively dark image.
In the meantime, the variation of luminance of 4 cd/m2 is a value visible as large difference in luminance and particularly, in a dark image, the variation seems remarkable.
Therefore, though 256 gradations are reproduced at any APL, there is a problem that gradation in a dark image is deteriorated in case APL is low and the dark image is recognized as an image poor in gradation.
To prevent such deterioration of gradation, there is also a method of complementing a gradation based upon an original signal and enhancing gradation based upon the information using a spatial and temporal method such as the error diffusion.
According to such a method, average gradation is improved, however, a problem that as emission of a large luminance step discretely occurs, an image seems rough is left.